It is highly desirable to use modular multi-level converters to create a regenerative drive. The modular multi-level topography provides a design which can produce high output voltages while using lower voltage level switching devices. One such topography is shown in FIG. 1 and in U.S. Pat. No. 5,625,545 which is hereby incorporated by reference. In this design, a plurality of series connected inverters or cells form a three-phase output for variable frequency control of the motor. The series connected outputs of each cell are fed from a three-phase rectifier. Multiple three-phase windings of a single transformer feed the rectifiers in each cell. The cells as shown have a rectifying section, a capacitor storage section, and an inverter output section. Harmonic components on the input side are effectively canceled by the phase relationship in the transformer windings. This type of drive has been demonstrated to be highly efficient and provides low harmonic content on the input while providing a very low harmonic content variable frequency AC output to a three-phase motor.
FIG. 2a shows a cascaded multilevel inverter for static VAR compensation/generation applications. The cascade multi-level inverter consists of single-phase full bridges, in which each bridge has its own separate DC source. The inverter of FIG. 2a can generate almost sinusoidal waveform voltages with each power semi conductor switching on and off only once per cycle. It can eliminate the need for transformers in a static VAR compensator. As shown in FIG. 2a, multiple inverters have their respective outputs connected in series. As shown, each H-bridge inverter unit generates a quasi-square waveform. As a result, the staircase voltage waveform shown in FIG. 2b is obtained. Switching phase angles are calculated off-line to minimize harmonics for each modulation index.
FIG. 3a shows a multicell voltage source inverter as could be used in an AC voltage motor drive. The series connection of cells shown in FIG. 3a is also shown in more detail in FIG. 3b. As seen in FIG. 3b, the cells 1 through 3 have their output voltages VC1, VC2, VC3 connected in series. As shown in FIG. 3b, the input to each cell is a single-phase transformer and the three inputs to cells 1 through 3 can be connected in parallel across the line.
It is highly desirable to have a low-cost variable frequency drive or power supply which gives an output at medium voltage or higher while providing low harmonics to the line and load. In addition, it would be desirable if the device can transfer real power between the input and the load in both directions, and have controllable VAR demand (leading or lagging) at the input.